IDs and credit cards

Are you always asked to show your ID when you pay with your credit card?
It makes me worried when I am not asked to, especially when it says on the back "picture ID required" and that gets ignored... Well... it does make me happy when I don't have an ID on me (which unfortunately happens regularily =( ...) But it is alarming! Anyone can use my credit card in case I lose it and don't notice that in time.